Coronary artery disease is one of the leading causes of death worldwide. The ability to better diagnose, monitor, and treat coronary artery diseases can be of life saving importance. Intravascular optical coherence tomography (OCT) is a catheter-based imaging modality that employs safe, non-ionizing near-infrared light to peer into coronary artery walls and present images valuable for the study of the vascular wall architecture. Utilizing broad-band coherent light, interferometry, and micro-optics, OCT can provide video-rate in-vivo tomography within a diseased vessel with resolution down to the micrometer level. This level of detail enables OCT to diagnose as well as monitor the progression of coronary artery disease.
The quantitative assessment of vascular pathology and its progression involves the calculation of different quantitative measures such as the vessel cross-sectional area, mean diameter, and blood flow resistance, all of which rely on the accurate identification of the luminal border. While the luminal border in OCT images is clearly identifiable by the human eye, it is tedious, expensive, and time consuming to manually trace the luminal border. Thus there is a need for a reliable technique that can automatically identify the luminal border.
OCT produces images that are higher in resolution and contrast compared to those of intravascular ultrasound (IVUS). As opposed to IVUS which images through blood, OCT images are typically acquired with blood cleared from the view of the optical probe. This is one reason the luminal border in OCT images is sharper and more defined compared to that in IVUS images.
Cross-sectional diameter and area measurements provide interventional cardiologists with useful guidance for stent sizing and placement. However, the relationship of these geometric measurements to clinically relevant variables, such as ability of the artery to supply an adequate flow of blood to the myocardium when metabolic demands are high, is not well understood. In early studies, the percent stenosis of an individual coronary lesion measured by angiography was found to be a relatively poor predictor of the physiological significance of the lesion. In contrast, several later studies demonstrated that lumen measurements made by quantitative coronary angiography (QCA) and IVUS correlate closely with physiologic measurements of coronary obstruction, including coronary flow reserve (CFR) and fractional flow reserve (FFR). For example, several studies found a high correlation between area stenosis, measured by QCA, and CFR measured by a Doppler flow wire. It appears that the standard angiographic (and IVUS) measures of lesion severity—the minimum lumen area (MLA), percentage stenosis, and lesion length—do indeed convey physiologically relevant information. However, the value of any single geometrical measure as an independent predictor of the physiological significance of a lesion in a wide patient population is not clear.
Several factors contribute to the limitation of standard angiographic and IVUS lumen measurements for assessment of the physiological significance of coronary stenoses. First, the accuracy and reproducibility with which cross-sectional areas can be measured with angiography, which generally has a spatial resolution of 0.2-0.4 mm, are relatively low. The angle of the X-ray projection, in addition to the shadowing effect of lesions with irregular contours, can increase errors significantly beyond the theoretical minimums. Even state-of-the-art IVUS imaging systems, which have resolutions of approximately 0.15 mm in the axial dimension and 0.3 mm in the angular dimension, cannot accurately measure the cross-sectional areas of small eccentric lesions or lesions with irregular borders.
Second, the hemodynamic effects of a lesion depend on local variations of its cross-sectional area integrated over the entire length of a lesion. Therefore, the minimum cross sectional area alone is insufficient to characterize the pressure drop across a lesion at a given flow rate, especially in patients with diffuse coronary disease.
Third, when assessing the physiological significance of a lesion and the potential value of revascularization, it is important to know the relative areas of the reference and stenotic segments, in addition to the absolute value of the minimum lumen area. No single geometrical measure in clinical use today conveys information about both percent stenosis and MLA.
Fourth, the flow resistance or pressure drop caused by an incremental segment of a lesion depends on its shape as well as its cross-sectional area and length. Especially at high blood flow rates, the eccentricity and local slope of the walls of the artery can influence the effective resistance of a lesion, because losses due to flow separation and turbulence depend on local flow velocity.
Finally, in certain patients, the flow reserve of the myocardium supplied by the vessel can be low, due to microvascular disease, flow through collateral branches, or capillary shunts within infarcted myocardium. Therefore, even if the vascular resistance of a lesion in the vessel is high, revascularization may be contraindicated, because the pressure drop across the lesion may be clinically insignificant.
Intravascular OCT imaging, applied in combination with new clinical parameters based on advanced analysis of lesion morphology, has the potential to overcome many of the limitations of conventional measures of lesion severity based on angiography and IVUS. The high resolution of OCT enables accurate measurement of the shape and dimensions of the vessel lumen over the length of the lesion and its adjacent reference segments. Furthermore, advanced models of flow dynamics enable the physiological significance of lesions to be estimated under both normal and hyperemic conditions. It should be realized, however, that the clinical value of quantitative lesion morphology measurements—even when accurate—may be limited by physiological conditions in certain patients. Finally, high-frequency OCT imaging has the advantage that it can precisely delineate three-dimensional contours of long segments of coronary arteries in a few seconds to assist cardiologists in their real-time diagnosis and treatment during PCI procedures.
In spite of advances in intravascular imaging, cardiologists frequently do not take full advantage of the capabilities of OCT and IVUS for planning and evaluating stent deployment, because the measurements currently derived from the images provide insufficient information to predict the effectiveness of treatment. For example, many cardiologists rely on minimum lumen area (MLA) as a key variable in their stenting decisions. If MLA measurements are sufficiently low, the cardiologist may decide to implant a stent. Based on the diameters and locations of reference vessel segments, the cardiologist must then choose the proper position, length, and diameter of the stent. The wrong choice of the size or location of the stent may lead to the failure to restore blood flow and may even cause potentially serious clinical complications, such as stent migration, stent thrombosis, or dissection of the vessel wall. There is a need for new methods for optimization of stent sizing and positioning based on measurements derived from intravascular images. To achieve maximum clinical benefit, these new methods should enable cardiologists to predict the potential physiological consequences of implanting stents of different diameters and lengths in different locations.
The present invention addresses these needs.